Dramatic shifts in public opinion about marijuana legalization are forcing both lawmakers and the public to deal with clashing federal and state policies.

In just weeks, three dispensaries will open in the Washington D.C. area.

Eighteen states and the District of Columbia have legalized the use of marijuana for medical purposes since California voters made the first move in 1996.

Voters in Colorado and Washington state took the next step last year and approved pot for recreational use. Alaska is likely to vote on the same question in 2014, and a few other states are expected to put recreational use on the ballot in 2016.

Nearly half of adults have tried marijuana, 12 percent of them in the past year, according to a survey by the Pew Research Center. More teenagers now say they smoke marijuana than ordinary cigarettes.

Fifty-two percent of adults favor legalizing marijuana, up 11 percentage points just since 2010, according to Pew. Sixty percent think Washington shouldn’t enforce federal laws against marijuana in states that have approved its use. Seventy-two percent think government efforts to enforce marijuana laws cost more than they’re worth.

Starting Monday, Vermonters will not be arrested for possessing less than an ounce of marijuana.

The decriminalization law, signed by Vermont Gov. Peter Shumlin (D) last month, will remove criminal penalties on small amounts of cannabis and replace them with civil fines.

According to the new measure, first-time offenders will not get more than a $200 fine for possession. The fine will increase for repeat offenders -- $300 for a second offense and $500 for every offense thereafter -- but, under the law, marijuana possession will no longer result in the creation of a criminal record.

Possession of more than an ounce of marijuana remains a criminal offense, and state law still forbids the cultivation of cannabis plants.

“This is a much-needed step forward toward a more sensible marijuana policy,” said Matt Simon, legislative analyst for the Marijuana Policy Project. “Nobody should be subjected to life-altering criminal penalties simply for possessing a substance that is objectively less harmful than alcohol.”

Vermont legalized medical marijuana in 2004, and now joins 14 other states that have adopted decriminalization laws for non-medical cannabis. In 2012, voters in two additional states, Washington and Colorado, approved ballot measures to tax and legalize marijuana for recreational use among those 21 years old and above.